Asymmetric measurement-based dynamic packet assignment system and method for wireless data services

ABSTRACT

A method and system for assigning downlink and uplink channels to a mobile station registered with a base station, which uses an interference-sensing scheme and which provides improved reliability and performance over conventional schemes.

Benefit is hereby claimed under 35 USC 119(e) of the U.S. ProvisionalApplication filed Mar. 23, 2000 and assigned Ser. No. 60/191,500.

FIELD OF THE INVENTION

The invention disclosed is related to measurement-based dynamic packetassignment for wireless data services.

RELATED ART

This application is related to U.S. Pat. No. 5,404,574 to M. Benveniste,for “Apparatus and Method for Non-Regular Channel Assignment in WirelessCommunication Networks,” and to U.S. Pat. No. 5,809,423 for“Adaptive-Dynamic Channel Assignment Organization System and Method.”

BACKGROUND OF THE INVENTION

The increased popularity of personal mobile communication and ofportable computing will significantly increase the demand for wirelesscommunication capacity, since they are heavy users of e-mail andweb-browsing applications. Access through fixed wireless communicationwill also contribute to this demand growth. Voice calls will increasethe circuit-switched traffic loads that have dominated wirelesscommunication up until now. The emerging convergence of voice and datatraffic into a single flow of IP packets will generate anever-increasing packet-switched traffic load. Resource utilizationefficiency is key to preserving the dynamic multiplexing advantagesachievable from this convergence. Regardless of its origin, theincreased demand will require more efficient methods of utilizing the RFspectrum allocated for this purpose.

Digital multiple access techniques must be optimized in order to meetthe growing demand for wireless communication capacity. The concern hereis with digital “channelized” methods, such as the North American timedivision multiple access (TDMA)/frequency division multiple access(FDMA) system defined by the IS-136 digital cellular standard or theEuropean TDMA/FDMA system defined by the Groupe Speciale Mobile (GSM)digital cellular standard. A channel in a TDMA/FDMA system is atime-slot. The objective in maximizing traffic-carrying capacity is toassign a channel to the unit of traffic load in a way that maximizes thesystem's throughput without violating quality-of-service (QoS)requirements. This is true whether it is a circuit-switched call thathas exclusive use of the circuit or a packet-switched call that sharesthe circuit with other calls.

New algorithms must be devised the meet an array of differentquality-of-service (QoS) requirements that arise with packet-switchedtraffic from diverse applications. Though the QoS requirements candiffer between different traffic types, their common objective andenvironmental/engineering constraints permit the use of similar channelassignment concepts for both. This is because the assignment of channelsto a packet in a packet-switched network resembles the assignment of achannel to a call for its duration in a circuit-switched network. Tounderscore this similarity, the terms packet and call are usedinterchangeably herein.

The notion of applying circuit-switched channel assignment concepts topacket-switched traffic should be pursued with care as there existfundamental differences. An important consideration relates to theasymmetry of the assignment in the two communication directions, theuplink direction from the mobile to the base station and the downlinkdirection from the base station to the mobile. With circuit-switchedvoice traffic, the uplink and downlink channels are paired, and channelassignment can thus utilize information available on only one direction.With packet-switched traffic, uplink and downlink channels would betypically independently assigned. A reason is the imbalance of thetraffic loads in the two directions: there is more traffic on thedownlink than on the uplink for certain applications, such as webbrowsing. But, even if the loads were balanced, channel assignmentshould occur independently in the two directions because of thenon-coincidence of uplink and downlink packets, even when occurringduring the same session.

Uncoupling the channel assignment for the two communication directionsaffects the applicability of circuit-switched traffic algorithms topacket-switched traffic. An important consideration relates to channelassignment algorithms relying on interference-sensing, known also asmeasurement-based algorithms. The problem to be addressed is how toperform asymmetric measurement-based channel assignment on a singledirection.

Another consideration in packet-switched channel assignment is thepacket length. Short “calls” require that the time dedicated to channelassignment be short in order to avoid capacity loss. To carry outchannel assignment efficiently in a multi-cell environment, the multiplebase stations must be synchronized. With packets of constant length, orof a small-integer multiple of a fixed length, channel selection bydifferent base stations will occur simultaneously. In that case thepossibility of contention for the same channel becomes likely.

I. Measurement Based Dynamic Channel Assignment (DCA)

There are two general forms of flexible channel assignment: adaptive,dynamic, and their combination known as adaptive-dynamic. See forexample “Channel Assignment Schemes for Cellular MobileTelecommunication Systems: A Comprehensive Survey”, I. Katzela and M.Naghshineh, IEEE Personal Communications, June 1996. Also see the abovereferenced U.S. Pat. No. 5,404,574 and U.S. Pat. No. 5,809,423. Adaptivechannel assignment (ACA) is a time-variable fixed channel assignment(FCA), whereby the set of channels allocated for use by a base stationis fixed over a time interval, typically longer than the call duration.Dynamic channel assignment (DCA) also involves allocated sets ofchannels, whose membership may vary in time. The difference between thetwo is that the channel sets in ACA do not overlap and, moreimportantly, all allocated channels can be used simultaneously withoutviolating the pre-specified quality-of-service target. With DCA, on theother hand, this requirement on the composition of channel sets is notobserved. In fact, a common DCA approach employs channel sets comprisingall the available channels. Consequently, before a channel is assignedto a call by DCA, a test is required to establish observance of the QoSrequirement. The channel-assignment admissibility (channel assignmentadmissibility (CAA) criteria employed in this test vary.

The channel assignment admissibility (CAA) criterion may rely either onreal-time channel utilization information shared across neighboring basestations and combined with knowledge of the interference relationshipsbetween base stations, or on real-time interference sensing. Dynamicchannel assignment (DCA) algorithms employing the latter criterion arereferred to as measurement-based algorithms. Such algorithms areparticularly attractive for distributed system architectures as theyrequire no real-time information sharing between base stations onchannel utilization.

A. Measurement-Based Channel Assignment Admissibility (CAA) Criterion

A measurement-based channel assignment admissibility (CAA) criteriondelivers the target quality of service only if the utilized measurementscan predict reliably whether the channel assignment under considerationwill violate the quality-of-service requirement. Many of the proposedmeasurement-based dynamic channel assignment (DCA) algorithms fail to doso. The IS 136 digital cellular system uses the mobile stations tomeasure the signals from surrounding base stations and report thosemeasurements back to the serving base station. This is primarily usedfor mobile assisted handoff so that the network can decide whether ahandoff is required. For channel assignment in an IS 136 digitalcellular system, the quality-of-service requirement will not be met whenthe serving base station selects a channel for an incoming voice call byrelying exclusively on measurements of the signal strength on downlinkchannels. Such measurements for channel assignment are calledMobile-Assisted Channel Assignment (MACA) measurements.

This situation is shown in FIG. 1, which depicts an indoor cellularsystem and an outdoor cellular system, respectively. In FIG. 1, mobilestation 101 (MS1) is registered on base station 102 (BS1) and mobilestation 103 (MS2) is registered on base station 104 (BS2). In FIG. 2,mobile station 201 (MS1) is registered on base station 202 (BS1) andmobile station 203 (MS2) is registered on base station 204 (BS2). Theindoor system employs omni-directional base stations (with full-aperturetransmitters and receivers) housed within a building whose walls, orother obstructions, may attenuate the transmitted signal. The outdoorsystem employs base stations with directional transmitters and receiversthat limit the coverage angle. In both systems, mobiles haveomni-directional receivers and transmitters. The common feature in thetwo systems is that a downlink signal from BS2 102 or 202 is hardlyperceptible by mobile MS1 101 or 201 because of the location of the two.An obstruction attenuates the signal in the indoor system, and themobile lies outside the beamwidth of the directional antenna of basestation BS2 in the outdoor system.

Suppose that Mobile MS2 103 or 203 is engaged in a call. Suppose alsothat a call is initiated by mobile MS1 101 or 201 and that the list ofchannels sent to mobile MS1 101 or 201 by base station B1 102 or 202 forMACA measurement includes the channel used by mobile MS2. Because of itslocation, the signal measured by mobile MS1 101 or 201 on the channelused by mobile MS2 103 or 203 will be sufficiently weak to lead to theassignment of that channel to the incoming call. This assignment willcause interference to mobile MS2 103 or 203, and hence violate thequality-of-service requirement, because the CAA criterion used wasinadequate. In general, a CAA criterion that relies on mobilemeasurements alone to sense interference will be inadequate.

A measurement-based channel assignment admissibility (CAA) criterionwould meet the QoS requirement if measurement of the interferencepotential of a channel assignment is made on the exact same path to betraversed by the signals resulting from the assignment. In general, twotypes of measurements are needed in order to clear a channel forassignment. A measurement clearing the path between the mobile and theneighboring base stations, and another between the serving base stationand the mobiles served by neighboring base stations. Clearance ofchannels can be accomplished differently depending on whether uplink anddownlink channels are paired or not.

II. Coupled Uplink and Downlink Channel Assignment

Channel assignment in circuit-switched wireless networks occurs inpre-defined pairs of uplink and downlink channels. Such circuit-switchedwireless networks are typically used to carry voice traffic. In suchsystems a measurement-based channel assignment admissibility (CAA)criterion could rely on a single signal strength measurement for bothchannels of the pair. See for example “Distributed Packet DynamicResource Allocation (DRA) for Wireless Networks”, J. F. Whitehead, Proc.of VTC '96, pp 111-115. That is, both the base station and the mobileare engaged in measurement of the signal strength of a candidate pair ofchannels. The mobile's measurement clears the downlink channel and itsassociated uplink channel along all paths between the mobile and theneighboring base stations. The base station's measurement clears theuplink channel and its associated downlink channel along the pathbetween the serving base station and the mobiles served by neighboringbase stations.

It can be seen in FIGS. 1 and 2 that the coupled measurements wouldprovide the indication of the interference potential for the channelpair. As explained earlier, a signal strength measurement by mobile MS1101 or 201 alone indicates that there is no call on the downlink channelused by mobile MS2 103 or 203 because the obstruction attenuates thesignal in the indoor system, and the mobile lies outside the beam-widthof the directional antenna of base station BS2. If, in addition tomobile MS1 101 or 201 measuring the signal strength on the downlinkchannel used in mobile MS2's 103 or 203 call, base station BS1 102 or202 also measured the signal strength on the paired uplink channel,there would be the indication that the pair of channels was used by aneighboring base station and hence the assignment would not be made.

III. Directionally-Uncoupled Channel Assignment

Unlike in circuit-switched wireless systems, channel assignment inpacket-switched wireless systems requires uncoupling along the uplinkand downlink communication directions, as the traffic in the twodirections is non-coincident. That is, packets from the mobile and thebase station occur at different times. Thus uncoupling would be requiredeven if the traffic loads in the two directions were reasonablybalanced.

In theory, both frequency-division and time-division duplex can supportuncoupled channel assignment between uplink and downlink. Time-divisionduplex allows more efficient channel training, and allocation of theradio resource between the two directions requires no planning. Withfrequency-division duplex, a different number of channels would be madeavailable in the two directions when the traffic loads along the twodirections differ. Additionally, the duration of the measurement andchannel selection must be short for, if it is significant relative tothe duration of a call, it would cause capacity loss.

The problem of accommodating unbalanced packet traffic loads by applyinga measurement-based channel assignment algorithm on a single directionhas been addressed in “An OFDM-Based High-Speed-Data (HSD) Air InterfaceProposal”, J. C. Juang and S. Timuri, AWS submission to the UniversalWireless Communications Consortium UWCC GTF HSD/97.10.0709, Nov. 11,1997. See also “Dynamic Packet Assignment for Advanced Internet CellularServices”, J. C. Chuang and N. R. Sollenberger, Proc. Of Globecom '97.See also “Advanced Cellular Internet Service (ACIS)”, L. J. Cimini, Jr.,J. C. Chuang, and N. R. Sollenberger, IEEE Communications Magazine,October 1998. A time division multiple access (TDMA)/frequency divisionmultiple access (FDMA) frame structure was proposed for downlink packetassignment, which was based on the Orthogonal Frequency DivisionMultiplexing (OFDM) technique for multi-carrier modulation. Pilot tonesthat correspond to the downlink traffic channels in use are transmittedsimultaneously by the base stations, thus enabling the mobiles to scanthe pilots and complete channel assignment quickly. Mobiles with pendingpackets measure the tones transmitted by the neighboring base stationsand report the list of interference-free channels to their serving basestations. The serving base station then notifies the mobiles of thechannel assigned to transmit the traffic packet. By staggering the timeof channel selection, the possibility of contention for the same channelby different base stations is avoided, which would be caused by theconcurrency of channel assignment among base stations. Interfering basestations do not engage in channel assignment at the same time.

While the above-described scheme satisfactorily addresses the issue ofmeasurement delay and contention, it does not always lead tointerference-free channel assignments. Interference can result becausethe path traversed by the signal from the serving base station to themobiles served by neighbor base stations, is not the same as the pathtraversed by the signal transmitted by a neighbor base station andsensed by the mobile during the interference measurement. As illustratedin FIGS. 1 and 2, there could be instances whereby mobile MS1 101 or 201which is served by base station BS1, does not receive the pilot tonesfrom a neighboring base station, base station BS2, even though the pilotis transmitted. This could be due to an obstruction, or simply to theorientation of a directional antenna. Hence, if base station BS1 selectsthe channel corresponding to the missed pilot tone, it could causeinterference to mobile MS2 103 or 203, which is served by base stationBS2.

Thus, a need arises for an interference-sensing scheme for use inasymmetric channel assignment which provides improved reliability andperformance over conventional schemes.

SUMMARY OF THE INVENTION

The present invention is a method and system for assigning downlink anduplink channels to a mobile station registered with a base station. Thepresent invention uses an interference-sensing scheme for use inasymmetric channel assignment which provides improved reliability andperformance over conventional schemes.

One embodiment of the present invention is a method of assigning adownlink channel to a mobile station registered with a base station.Pilot tones being transmitted by a plurality of active mobile stationsregistered with the base station are turned off. Each turned off pilottone corresponds to an assigned downlink channel. The mobile station ispaged from the base station with a pending traffic packet. Interferencesensing is performed at the base station to identify interference-freedownlink channels. A downlink traffic channel is assigned at the basestation to the mobile station to receive the pending packets and thedownlink channel assignment is transmitted from the base station to themobile station. The downlink channels may be assigned to a plurality ofmobile stations registered with the base station. There may be aplurality of base stations and the method may be performed successivelyfor each of the plurality of base stations.

Another embodiment of the present invention is a method of assigning anuplink channel to a mobile station registered with a base station. Pilottones being transmitted by the base station are turned off. Each turnedoff pilot tones corresponds to an uplink channel assigned to one of aplurality of active mobile stations registered with the base station.The mobile station requests access for a traffic packet. Interferencesensing is performed at the plurality of active mobile stations toidentify interference-free uplink channels. A list of uplink channelsidentified as being acceptably interference-free is transmitted fromeach one of the plurality of active mobile stations. An uplink channelis assigned to the mobile station by the base station and the uplinkchannel assignment is transmitted from the base station to the mobilestation. The uplink channels may be assigned to a plurality of mobilestations registered with the base station. There may be a plurality ofbase stations and the method may be performed successively for each ofthe plurality of base stations.

Another embodiment of the present invention is a method of assigning anuplink channel and a downlink channel to a mobile station registeredwith a base station. Pilot tones being transmitted by a plurality ofactive mobile stations registered with the base station are turned off.Each turned off pilot tone corresponds to an assigned downlink channel.Pilot tones being transmitted by the base station are turned off. Eachturned off pilot tones corresponds to an uplink channel assigned to oneof a plurality of active mobile stations registered with the basestation. The mobile station is paged from the base station with apending traffic packet. The mobile station requests access for a trafficpacket. Interference sensing is performed at the base station toidentify interference-free downlink channels. Interference sensing isperformed at the plurality of active mobile stations to identifyinterference-free uplink channels. A list of uplink channels identifiedas being acceptably interference-free is transmitted from each one ofthe plurality of active mobile stations. A downlink traffic channel isassigned at the base station to the mobile station to receive thepending packets. An uplink channel is assigned to the mobile station atthe base station. The downlink channel assignment and the uplink channelare transmitted from the base station to the mobile station. Thedownlink channels and uplink channels may be assigned to a plurality ofmobile stations registered with the base station. There may be aplurality of base stations and the method may be performed successivelyfor each of the plurality of base stations.

The resulting invention provides a reliable measurement-based channelassignment admissibility (CAA) system and method which is applicable tounidirectional uplink or downlink channel assignment.

DESCRIPTION OF THE DRAWINGS

The present invention will be more fully appreciated by referring to theaccompanying drawings.

FIG. 1 is a depiction of a prior-art indoor cellular system.

FIG. 2 is a depiction of a prior-art outdoor cellular system.

FIG. 3 is an exemplary block diagram of a wireless telecommunicationssystem, in which the present invention may be implemented.

FIG. 4 is a flow diagram of a process of operation of the presentinvention.

FIG. 5 is a flow diagram of a downlink channel assignment process,according to the present invention.

FIG. 6 is an exemplary format of a superframe, in which the processshown in FIG. 5 is carried out.

FIG. 7 is a flow diagram of an uplink channel assignment process,according to the present invention.

FIG. 8 is an exemplary format of a superframe, in which the processshown in FIG. 7 is carried out.

FIG. 9 is a flow diagram of a downlink channel and uplink channelassignment process, according to the present invention.

FIG. 10 is an exemplary format of a superframe, in which the processshown in FIG. 9 is carried out.

FIG. 11 illustrates a frame structure for directionally uncoupledchannel assignment, in accordance with the invention.

FIG. 12 illustrates a hexagonal base station layout.

FIG. 13 illustrates a cloverleaf base station layout.

FIG. 14 is an example of a wireless telecommunications system, accordingto the present invention.

DISCUSSION OF THE PREFERRED EMBODIMENTS

An exemplary wireless telecommunications system 300, in which thepresent invention may be implemented, is shown in FIG. 3. System 300includes a plurality of wireless base stations, such as base stations302A through 302N. Each base station serves, or has registered with it,a plurality of mobile stations. For example, base station 302A hasregistered with it mobile stations 304A-1 through 304A-N, base station302B has registered with it mobile stations 304B-1 through 304B-N, andbase station 302N has registered with it mobile stations 304N-1 through304N-N. Each base station communicates with the mobile stationsregistered with that base station over wireless links. For example, basestation 302A communicates with mobile station 304A-1 over wireless links306 and 308 and with mobile station 304N over wireless links 310 and312. Links 306 and 312, which communicate from the mobile stations304A-1 and 304A-N, respectively, to base station 302A are termeduplinks. Links 308 and 310, which communicate from base station 302A tothe mobile stations 304A-1 and 304A-N, respectively, are termeddownlinks. Calls and/or packets, which terms will be usedinterchangeably hereinafter, are transmitted from the mobile station tothe base station over the uplink, while calls and/or packets aretransmitted from the base station to the mobile station over thedownlink.

A. Requirements of a Measurement-Based Channel Assignment Admissibility(CAA) Criterion

If interfering base stations are engaged in channel assignmentconcurrently, it is possible that they would select the same channel.Staggering the time when measurement and channel selection decisions aremade by interfering base stations, would avoid contention. A reliablechannel assignment admissibility (CAA) criterion is obtained foruncoupled channel assignment by returning to the basic requirement: thatall the paths along which the assigned channel will traverse must becleared.

1. Downlink Communication Direction

Considering downlink channel assignment first, the channel assignmentadmissibility (CAA) criterion would require that

-   (i) the signal transmitted on the candidate downlink channel be    received by the mobile without interference, and-   (ii) the channel assignment cause no interference to active    neighboring mobiles.

To satisfy the first point, the mobile with a pending packet must clearthe candidate downlink channels by measuring the received signalstrength and communicate this information to the serving base station.If the signal is weak, the first criterion will be met. In order toestablish the requirement (i) for the downlink CAA criterion one coulduse a feature like MACA (mobile assisted channel assignment) availablein the IS 136 and the GSM air interface standard. Paged mobiles (mobileswith packets pending) would scan candidate downlink channels and measurethe received signal strength. Alternatively, when channel reservationand data transmission are separated, the base station could transmitsimultaneously pilot tones that correspond to its assigned downlinktraffic channels F. Furuya and Y. Akaiwa, “Channel segregation, adistributed adaptive channel allocation scheme for mobile communicationssystems”, Trans. IEICE, Vol. E74, June 1991, pp. 1531-1537. Page mobilesscan the transmitted pilots to identify the acceptable downlinkchannels, of which they would inform their base station. A list, ratherthan the best channel, is returned to the serving base station, in casemore than one mobile report the same channel and in order to accommodatedifferent bit rates through time-slot pooling.

In order to meet criterion (ii), it is necessary to establish whether anactive mobile that can be reached at sufficient intensity by a signalemanating from the serving base station, uses the candidate channel. Inaccordance with the invention, the following two equivalent conditionsare established:

-   -   (ii.1) whether there are any active mobiles that can be reached        by a signal from the serving base station; and    -   (ii.2) whether any of the mobiles so identified use the        candidate downlink channel.

The first question can be answered by having the serving base stationengage in a measurement of a signal transmitted by the active mobiles onthe uplink control channel. By the symmetry found in point-to-pointconnections, a strong uplink received signal would indicate that thesignal on the reverse path would be also strong. The second question canbe answered by requiring the control signal to indicate the identity ofthe downlink channel used by the mobile. For instance, the activemobiles (mobiles with pending packets that have already been assigned achannel) transmit on the uplink control channel pilot tones thatcorrespond to their assigned downlink channel. The serving base stationscans the pilots to obtain a measure of both the proximity and, hence,the interference potential from (and to) nearby active mobiles and ofthe downlink channels these mobiles use. The base station assigns adownlink channel by eliminating from the list of acceptable channelsreceived from the paged mobile the downlink channels already assigned tonearby active mobiles.

2. Uplink Communication Direction

The channel assignment admissibility (CAA) criterion for uplink channelassignment is expressed similarly as the requirement that

-   (i) the signal transmitted on the candidate uplink channel be    received by the base station without interference, and-   (ii) the channel assignment cause no interference to neighboring    base stations.

To answer the first question, it is sufficient for the serving basestation to clear the candidate uplink channels by measuring the receivedsignal. If the signal is weak, the first criterion will be met. Theserving base station scans the uplink traffic channels and measuresinterference. This could be readily achieved with existing systems byadding a radio at the base station cite. Alternatively, narrow bandtones corresponding to the uplink channels used by active mobiles aretransmitted by all such mobiles and scanned by the serving base station.A list of acceptable uplink channels is thus constructed.

In order to meet criterion (ii), it is necessary to establish whether abase station that can be reached at sufficient intensity by a signalfrom the mobile with a pending packet uses the candidate uplink channel.In accordance with the invention, the following two equivalentconditions are established:

-   -   (ii.1) which base stations can be reached by a signal from the        mobile with the channel request; and    -   (ii.2) whether any of the base stations so identified use the        candidate uplink channel.

The first question can be answered by having the paged mobile measurethe signal transmitted on the downlink control channel by theneighboring base stations. As before, because of the symmetry inpoint-to-point connections, a strong downlink received signal wouldindicate that the signal on the reverse path would be also strong. Thesecond question can be answered by requiring each base station toindicate on its control the identity of the uplink channels used by themobiles served by that base station. For instance, the base stationtransmits on a downlink control channel pilot tones that correspond totheir assigned uplink channels. A paged mobile scans the pilots toobtain a measure of both its proximity and, hence, the interferencepotential from (and to) neighboring base stations and of the uplinkchannels nearby base stations use. The list of noisy uplink channels isconveyed to the serving base station. The base station assigns an uplinkchannel by eliminating from the list of acceptable channels the list ofnoisy channels received from the paged mobile.

The interference-sensing scheme of the present invention identifieschannel use by neighboring cells reliably. A process of operation of thepresent invention is shown in FIG. 4. The process begins with step 402,in which all active mobile stations transmit pilot tones on the uplink.Each active mobile station transmits a tone corresponding to thedownlink channel it has been assigned. In step 404, the base stationsmonitor the pilot tones transmitted by the mobile stations. In step 406,the mobile stations covered by a given base station will turn off theirpilot tones in the assignment frame, which occurs on a staggeredschedule for each base station. This allows each base station todetermine the cell location of each mobile station whose pilot tone thebase station is receiving, and thus determine the channel usage ofmobile stations in neighboring cells. In step 408, using the informationrelating to channel usage of mobile stations in neighboring cells, eachbase station selects interference-free channels for its mobile stations.In step 410, each base station transmits the channel assignments to eachof its mobile stations.

B. Implementation of Proposed Interference-Sensing Scheme

The above schemes can be implemented with a variety of control/signalingmechanisms. The implementation method would depend on the type oftraffic, that is, the length of the packet or call and the duration ofthe measurement and channel selection relative to the packet length.Shorter packets would require faster measurement and channel selectionmechanisms. A description of the inventive method is presented for shortpacket traffic.

1. Downlink Channel Assignment

In order to establish condition (i) for the downlink channel assignmentadmissibility (CAA) criterion one can use a feature like mobile assistedchannel assignment (MACA) available in the IS 136 and the GSM airinterface standard. Paged mobiles (mobiles with packets pending) scancandidate downlink channels and measure the received signal strength.Alternatively, in order to reduce measurement delay, the base stationcould transmit simultaneously pilot tones that correspond to its activedownlink traffic channels. See the L. J. Cimini, Jr., et al. referencecited above. Paged mobiles scan the transmitted pilots to identify theacceptable downlink channels, of which they would inform their basestation. A list, rather than the best channel, is returned to theserving base station, in case more than one mobile report the samechannel and in order to accommodate different bit rates throughtime-slot pooling.

Criterion (ii) can be established by requiring the active mobiles(mobiles with pending packets that have already been assigned a channel)to transmit on an uplink control channel pilot tones that correspond totheir assigned downlink channel. The serving base station scans thepilots to obtain a measure of both the proximity and, hence, theinterference potential from (and to) nearby active mobiles and of thedownlink channels these mobiles use. The base station assigns a downlinkchannel by eliminating from the list of acceptable channels receivedfrom the paged mobile the downlink channels already assigned to nearbyactive mobiles.

A process of channel assignment, according to the present invention,which is applicable to assign downlink channels for one-way traffic fromthe base station to one or more mobile stations, is shown in FIG. 5. Theprocess begins with step 502, in which the active mobile stationsregistered with the base station turn off the pilot tones correspondingto their assigned downlink channels. In step 504, the base station pagesthe registered mobiles with pending traffic packets. In step 506, thebase station performs interference sensing to identify interference-freedownlink channels. In step 508, the base station assigns downlinktraffic channels to the paged mobiles to receive their pending packets.In step 510, the base station transmits the channel assignments to themobile stations. In all other frames of a superframe the active mobilesregistered with the base station will transmit their pilot tonescorresponding to the assigned downlink channels.

Each base station is assigned a frame within each superframe, in whichthe process shown in FIG. 5 is carried out. Since the mobiles served byinterfering base stations turn off their pilots in the different frameswithin a superframe, the number of frames per superframe must be equalto at least the re-use factor needed to deliver the desired reception.An exemplary frame structure, according to the present invention, isshown in FIG. 6. The depicted frame structure assumes that there isone-way traffic from the base station to the mobile. Frame structure 600is an example of a downlink frame structure, while frame structure 602is an example of an uplink frame structure.

Downlink frame structure 600 includes a plurality of superframes, suchas super-frames 604A to 604Z. Each superframe includes a plurality ofassignment frames, such as assignment frame 606, which is the assignmentframe for an exemplary base station, designated BS1. Each assignmentframe is used to determine channel assignments for each neighboring basestation. Each assignment frame includes a plurality of segments, such assegments 607 and 608, in which transmissions from the base station toone or more mobile stations occur. For example, in segment 607, basestation BS1 pages the active mobile stations registered with basestation BS1, as in step 504 of FIG. 5. In segment 608, base station BS1transmits the selected downlink channels to the mobile stations, as instep 510 of FIG. 5.

Uplink frame structure 602 includes a plurality of superframes, such assuper-frames 610A to 610Z. Each superframe includes a plurality ofassignment frames, such as assignment frame 612, which is the assignmentframe for an exemplary base station, designated BS1. Each assignmentframe is used to determine channel assignments for each neighboring basestation. Each assignment frame includes a plurality of segments, such assegment 614, in which transmissions from one or more mobile stations tothe base station occur. For example, in segment 614, the mobile stationsregistered with base station BS1 stop transmitting their pilot tones, asin step 502 of FIG. 5, and the mobile stations of base stations otherthan base station BS1 transmit their pilot tones.

2. Uplink Channel Assignment

Uplink channel assignment is similar to downlink channel assignment.Condition (i) for the uplink channel assignment admissibility (CAA)criterion would require the serving base station to scan the uplinktraffic channels and measure interference. This could be readilyachieved with existing systems by adding a radio at the base stationcite. Alternatively, narrow band tones corresponding to the uplinkchannels used by active mobiles are transmitted by all such mobiles andscanned by the serving base station. A list of acceptable uplinkchannels is thus constructed.

Criterion (ii) can be established by requiring the base stations totransmit on a downlink control channel pilot tones that correspond totheir assigned uplink channels. A paged mobile scans the pilots toobtain a measure of both its proximity and, hence, the interferencepotential from (and to) neighboring base stations and of the uplinkchannels nearby base stations use. The list of noisy uplink channels isconveyed to the serving base station. The base station assigns an uplinkchannel by eliminating from the list of acceptable channels the list ofnoisy channels that was received from the paged mobile.

In general, when one-way traffic from the mobile to the base stations isinvolved, the roles of the base stations and mobiles are reversed. Aprocess of channel assignment, according to the present invention, whichis applicable to assign uplink channels for one-way traffic from one ormore mobile stations to the base station, is shown in FIG. 7. Theprocess begins with step 702, in which the base station turns off itspilot tones corresponding to the assigned uplink channels. In step 704,the mobile stations registered with base station BS1, which aregenerating traffic packets, request access for those packets. In step706, the active mobile stations engage in interference sensing toidentify interference-free uplink channels. In step 708, each activemobile station transmits to the base station its list of uplink channelsthat it identified as being acceptably interference-free uplinkchannels. In step 710, the base station examines the received lists ofchannels, selects channels that are overall acceptablyinterference-free, and assigns uplink channels to the mobile stationsthat requested access in step 704. In step 712, the base stationtransmits the channel assignments to the mobile stations that requestedaccess. In all other frames of a superframe the base station willtransmit its pilot tones corresponding to the assigned uplink channels.As described above, since the interfering base stations do not turn offtheir pilot tones in the same frame, the number of frames per superframemust be at least equal to the re-use factor needed to deliver thedesired reception quality.

Each base station is assigned a frame within each superframe, in whichthe process shown in FIG. 7 is carried out. An exemplary framestructure, according to the present invention, is shown in FIG. 8. Thedepicted frame structure assumes that there is one-way traffic from amobile station to the base stations. Frame structure 800 is an exampleof a downlink frame structure, while frame structure 802 is an exampleof an uplink frame structure.

Downlink frame structure 800 includes a plurality of superframes, suchas super-frames 804A to 804Z. Each superframe includes a plurality ofassignment frames, such as assignment frame 806, which is the assignmentframe for an exemplary base station, designated BS1. Each assignmentframe is used to determine channel assignments for each neighboring basestation. Each assignment frame includes a plurality of segments, such assegments 807 and 808, in which transmissions from base stations to oneor more mobile stations occur. For example, in segment 807, base stationBS1 turns off its pilot tones corresponding to the assigned uplinkchannels, while base stations other than BS1 continue to transmit theirpilot tones, as in step 702 of FIG. 7. In segment 808, base station BS1transmits the assigned uplink channels to the mobile stations, as instep 712 of FIG. 7.

Uplink frame structure 802 includes a plurality of superframes, such assuper-frames 810A to 810Z. Each superframe includes a plurality ofassignment frames, such as assignment frame 812, which is the assignmentframe for an exemplary base station, designated BS1. Each assignmentframe is used to determine channel assignments for each neighboring basestation. Each assignment frame includes a plurality of segments, such assegments 814 and 816, in which transmissions from one or more mobilestations to the base station occur. For example, in segment 814, themobile stations registered with base station BS1, which are generatingtraffic packets, request access for those packets, as in step 704 ofFIG. 7. In segment 816, each active mobile station transmits to the basestation its list of uplink channels that it identified as beingacceptably interference-free uplink channels, as in step 708 of FIG. 7.

II. Medium Access Control for Packet Traffic

In accordance with the invention, a scheme for uncoupling the channelassignment of uplink and downlink traffic is presented. Additionalconsiderations are posed by packet traffic, namely the need for fastchannel assignment decisions and the avoidance of contention. Certainconcepts can be borrowed from the interference-sensing scheme proposedfor downlink channel assignment in the L. J. Cimini, Jr., et al.reference cited above, as it addresses some of these issuessatisfactorily. The invention differs from that scheme by eliminatingthe interference that would occur occasionally due to insufficientmeasurement data, the introduction of some equity features, and QoSawareness. Additionally, in accordance with the invention, aninterference-sensing scheme for uplink channel assignment is provided.

A. The Method

In accordance with the invention, a medium access control scheme isprovided for uncoupled downlink and uplink channel assignment thataccommodates packet of fixed length (as with ATM) or variable length (aswith IP). the invention works with either frequency-division andtime-division multiplex techniques.

In accordance with the invention, narrow-band pilot tones are used,corresponding to assigned traffic channels, as seen in the L. J. Cimini,Jr., et al. reference cited above. Such tones can be scanned quickly andconveniently with OFDM modulation. While using strong synchronizationbetween base stations and their mobiles, scanning of the pilot tones isstaggered in order to avoid contention for the same channel betweenmutually interfering base stations with concurrent traffic. A basestation is assigned within each super-frame a frame, referred to as theassignment frame, during which it transmits its tones. Mobiles scan thetones and report a list of channels with low interference to the servingbase station. The base station selects a channel and notifies themobile.

The interference-sensing scheme in the L. J. Cimini, Jr., et al.reference cited above, is modified as follows. First, in addition torequiring the base stations to transmit tones corresponding to thedownlink channels acquired for their use, the active mobiles also do thesame on the uplink. That is, all active mobiles transmit a tonecorresponding to the downlink channel each mobile has been assigned.And, in addition to the mobiles engaging in signal measurement (andtransmission of the list of admissible channels to the base station),the serving base station will also listen to the tones transmitted bythe active mobiles. In order to be able to stagger channel-assignmenttimes, the active mobiles will turn off their pilots during their basestation's assignment frame, just like the base station will turn off itspilot tones during its channel assignment frame so that mutuallyinterfering base stations do not select channels concurrently. From itsmeasurements, the base station thus constructs its own list ofadmissible channels. Eliminating the channels absent from the basestation's list of admissible channels will shorten the list ofadmissible channels received from the paged mobile.

An analogous interference-sensing scheme is proposed for uplink channelassignment. Instead of transmitting tones that correspond to assigneddownlink channels, all the base stations and their active mobilestransmit tones corresponding to the assigned uplink channels.

In theory, both frequency-division and time-division duplex can supportuncoupled channel assignment between uplink and downlink. Withfrequency-division duplex, a different number of channels would be madeavailable in the two directions when the traffic loads along the twodirections differ.

A process of channel assignment, according to the present invention,which is applicable to assign uplink channels for asymmetric two-waytraffic between one or more mobile stations and the base station, isshown in FIG. 9. The process begins with step 902, the active mobilestations registered with the base station turn off the pilot tonescorresponding to their assigned downlink channels, while active mobilestations registered with base stations other than that base stationcontinue transmitting their pilot tones. In step 904, the base stationturns off its pilot tones corresponding to the assigned uplink channels.In step 906, the base station pages the registered mobiles with pendingtraffic packets. In step 908, the mobile stations registered with basestation BS1, which are generating traffic packets, request access forthose packets. In step 910, the base station performs interferencesensing to identify interference-free downlink channels. In step 912,the active mobile stations engage in interference sensing to identifyinterference-free uplink channels. In step 914, each active mobilestation transmits to the base station its list of uplink channels thatit identified as being acceptably interference-free uplink channels. Instep 916, the base station assigns downlink traffic channels to thepaged mobiles to receive their pending packets. In step 918, the basestation examines the received lists of channels, selects channels thatare overall acceptably interference-free, and assigns uplink channels tothe mobile stations that requested access in step 904. In step 920, thebase station transmits the channel assignments to the mobile stationsthat requested access. In all other frames of a superframe the activemobile stations registered with the base station will transmit theirpilot tones corresponding to the assigned downlink channels and the basestation will transmit its pilot tones corresponding to the assigneduplink channels. As described above, since the interfering base stationsdo not turn off their pilot tones in the same frame, the number offrames per superframe must be at least equal to the re-use factor neededto deliver the desired reception quality.

Each base station is assigned a frame within each superframe, in whichthe process shown in FIG. 9 is carried out. An exemplary framestructure, according to the present invention, is shown in FIG. 10. Thedepicted frame structure assumes that there is two-way traffic from amobile station to the base stations. Frame structure 1000 is an exampleof a downlink frame structure, while frame structure 1002 is an exampleof an uplink frame structure.

Downlink frame structure 1000 includes a plurality of superframes, suchas super-frames 1004A to 1004Z. Each superframe includes a plurality ofassignment frames, such as assignment frame 1006, which is theassignment frame for an exemplary base station, designated BS1. Eachassignment frame is used to determine channel assignments for eachneighboring base station. Each assignment frame includes a plurality ofsegments, such as segments 1006, 1007, 1008, and 1009, in whichtransmissions from base stations to one or more mobile stations occur.For example, in segment 1006, base station BS1 pages the active mobilestations registered with base station BS1, as in step 906 of FIG. 9. Insegment 1007, base station BS1 turns off its pilot tones correspondingto the assigned uplink channels, while base stations other than BS1continue to transmit their pilot tones, as in step 904 of FIG. 9. Insegment 1008, base station BS1 transmits the assigned uplink channels tothe mobile stations, as in step 920 of FIG. 9. In segment 1009, basestation BS1 transmits the assigned downlink channels to the mobilestations, as in step 920 of FIG. 9.

Uplink frame structure 1002 includes a plurality of superframes, such assuper-frames 1010A to 1010Z. Each superframe includes a plurality ofassignment frames, such as assignment frame 1012, which is theassignment frame for an exemplary base station, designated BS1. Eachassignment frame is used to determine channel assignments for eachneighboring base station. Each assignment frame includes a plurality ofsegments, such as segments 1014, 1016, and 1018, in which transmissionsfrom one or more mobile stations to the base station occur. For example,in segment 1014, the mobile stations registered with base station BS1,which are generating traffic packets, request access for those packets,as in step 908 of FIG. 9. In segment 1016, the active mobile stationsregistered with the base station turn off the pilot tones correspondingto their assigned downlink channels, while active mobile stationsregistered with base stations other than that base station continuetransmitting their pilot tones, as in step 902 of FIG. 9. In segment1018, each active mobile station transmits to the base station its listof uplink channels that it identified as being acceptablyinterference-free uplink channels, as in step 914 of FIG. 9.

B. The Time-Division Duplex Method

A frame structure description for uncoupled channel assignment ispresented in the context of a time-division duplex arrangement. Eitherdownlink or uplink channels can be assigned by the proposed scheme,which can also accommodate the uncoupled channel assignment on the twodirections concurrently. The proposed scheme is described in the contextof the latter. Time-division duplex allows the allocation of the radioresource between the two directions without the need for planning.Moreover, it can achieve more efficient channel training.

The concepts presented here can be used with protocols for fixed lengthpackets (like ATM packets) or for variable-length packets (like IPpackets). If the traffic involves variable-length packets, which aremultiple time-slots in length, several channels (time slots) may bereserved within the same super-frame in order to achieve higher user bitrates. It is also possible to achieve continuous transmission of apacket over several super-frames without the need for channelreselection. The extent to which these options are exercised will dependon the QoS priority of the packet stream, inter-stream synchronization,and other traffic management considerations. As these options are thepurview of the particular channel assignment algorithm employed, theirimplementation is left out of this discussion.

Each super-frame comprises a set of assignment frames to be used forchannel assignment/reservation purposes, plus additional segments to beused for reserved and unreserved traffic, in either direction. Each basestation is scheduled to perform channel assignment during one of theassignment frames. In subsequent assignment frames, the base stationhelps coordinate the channel assignment process of its neighbor basestations. Specifically, in frames following its scheduled assignmentframe, a base station will transit pilot tones corresponding to thechannels that have already been reserved for its use for thatsuper-frame. The reserved channels could be either uplink channels ordownlink, or both if a base station is handling both types on traffic ina super-frame. The pilot tones of the reserved channels will continue tobe transmitted over all super-frames for which a channel has beenreserved. In all assignment frames other than the scheduled assignmentframe, the active mobiles of a base station (which are the mobiles thathave already been paged and undergone channel assignment for the presentsuper-frame) will transmit tones corresponding to the channel(s)reserved for their respective use.

As illustrated in FIG. 11, the assignment portion of the super-frameconsists of alternating downlink and uplink segments, which areseparated by an idle time interval to allow the radios to switch fromtransmitting to receiving. An assignment frame comprises a downlinkpaging sub-frame, a downlink measurement sub-frame, followed by anuplink measurement sub-frame, and an uplink reporting sub-frame, thenfollowed by a downlink notification sub-frame.

During the assignment frame of a base station, the following controlfunctions will be carried out:

-   the base station turns off its pilots;-   the active mobiles served by the base station turn off their pilot    tones;-   the base station pages its registered mobiles with pending traffic    packets on the downlink paging sub-frame;-   the paged mobiles are engaged in interference sensing on the    downlink measurement sub-frame to identify interference-free    channels;-   the base station engages in interference sensing on the uplink    measurement sub-frame to identify interference-free channels;-   the list of acceptable channels are reported by the paged mobiles to    the base station on the uplink reporting sub-frame; and-   the base station assigns traffic channels to the paged mobiles on    which to receive their pending packets and broadcasts their identity    on the downlink notification sub-frame.    III. Performance of the Medium Access Control Scheme

The performance of the proposed interference-sensing scheme for downlinkand uplink channel assignment is measured in terms of the drop in theS/I ratio or the increase in the retransmission probability the proposedscheme would help prevent when compared to a scheme that employs mobilemeasurements only. A system of sectorized cells is considered, eachhaving three sectors with a 120-degree beam-width with a front-to-backratio of 20 dB.

Two layouts are considered. One is a hexagonal pattern, as illustratedin FIG. 12. sector D1 might re-use the channels used by sectors B1 andE1 since the mobile performing interference measurements would notreceive a sufficiently strong signal from these sectors. Conversely, amobile in sector E1 could receive co-channel interference contributed bysectors A1 D1, I1, and L1. Moreover, if the mobiles active in thecoverage area of sectors B1 and J1 are shielded from the signal fromsector E1, sectors B1 and J1 could also contribute to the interferenceat mobile m. Assuming an interference limited system, a propagation losscoefficient of 4, and a measured S/I ratio of 10 dB, the S/I ratiopenalty experienced by a mobile at location m in the coverage area ofsector E1 could be as high as 4.25 dB, as shown in Appendix I.

Also a system arranged in a cloverleaf pattern is considered, asillustrated in FIG. 13. In this situation sector H1 might re-use thechannels used by sectors D1, E1, and I1 since the mobile performinginterference measurements would not receive a sufficiently strong signalfrom these sectors. Conversely, a mobile in sector E1 could receiveco-channel interference contributed by sectors D1, I1, and H1. The S/Iratio penalty experienced by a mobile at location m in the coverage areaof sector E1 could be as high as 3.99 dB for this arrangement, as shownin Appendix I.

An S/I ratio penalty of such magnitude would cause a word-error-rateexperienced in packet transmission, and packet re-transmissionprobability, higher by an order of magnitude for the OFDM signalanalyzed in the L. J. Cimini, Jr., et al. reference cited above, whenemploying four transmit and two receive antennas for 40-μsec delayspread, 10-Hz Doppler, and QPSK with differential detection and ½-rateReed-Solomon coding.

One might be tempted to dismiss this result because location m is justone location and the impact on the average S/I ratio [or otherstatistical metric] might be less severe. To this one would rebut thatthe interference penalty realized at other locations may be either loweror higher depending on the power control policy employed and the rulesfor channel assignment, typically prescribed by the dynamic channelassignment algorithm implemented in conjunction with interferencesensing. For instance, consider the case where both Sectors D1 and I1assigned the same channel as that assigned to a mobile served by SectorE1 to transmit packets to mobiles located on their respective cellboundaries; namely locations n and l, respectively. If the power controlpolicy equalized the received signal strength, most mobile locations inSector E1 could experience a lower S/I ratio than location m as they arecloser to base stations D and I. Different scenarios could be describedwhereby location m would experience the lowest S/I ratio of alllocations in Sector E1.

But even if all other locations in Sector E1 experienced a higher S/Iratio than location m, and as a result the S/I statistics across thecell are affected less severely by an interference-sensing scheme thatmisses some times, that would not be relevant to users of fixed wirelesssystems, for instance. They stay in the same position, alwaysexperiencing the same performance. Unlike with mobile users, whosemovement to different locations will result in a better averageperformance, the higher S/I ratio experienced elsewhere in the cell isirrelevant to the fixed-wireless user. The same applies to portablecomputing users if they are likely to select their point of connectionbased on factors other than signal quality.

Of course, it is possible to design heuristic channel assignmentalgorithms that compensate for the missing interference information. Forexample, measurement-based dynamic channel assignment combined withchannel segregation, See F. Furuya and Y. Akaiwa, “Channel segregation,a distributed adaptive channel allocation scheme for mobilecommunications systems”, Trans. IEICE, Vol. E74, June 1991, pp.1531-1537. There, re-use patterns are determined from empiricallyobserved transmission successes and failures, helps avoid situationslike those described in this section. See the L. J. Cimini, Jr., et al.reference cited above. Greater efficiency would be achieved, however, ifco-channel interference was avoided reliably (through the use of theproposed interference-sensing scheme) and the channel assignmentalgorithm was selected to better focus on maximizing throughput.

The performance of the interference-sensing scheme of the presentinvention may be quantified in terms of the deterioration of the qualityof service requirement it helps prevent. For example, in the case inwhich the CAA criterion employed for downlink channel assignment reliesonly upon mobile measurements, the result would be substantialinterference increase. An example of base stations arranged in ahexagonal pattern is shown in FIG. 14. FIG. 14 shows a plurality of basestations 1101-1112, each base station having three sectors with a120-degree beamwidth. For example, base station 1101 has three sectors1101-1, 1101-2, and 1101-3. If the front-to-back ratio is 20 dB, sector1104-1 could re-use the channels used by sectors 1102-1 and 1105-1 sincethe mobile stations performing interference measurements would notreceive a sufficiently strong signal from these sectors. Conversely, amobile in sector 1105-1 could receive co-channel interferencecontributed by sectors 1101-1, 1104-1, and 1109-1.

Assuming an interference limited system, a propagation loss coefficientof 4, and a target S/I ratio of 10 dB, the S/I ratio penalty experiencedby a mobile station in the coverage area of sector 1105-1 could be ashigh as 3 dB. A penalty of such magnitude would cause theword-error-rate experienced in packet transmission, and the packetre-transmission probability, to increase by an order of magnitude whenemploying four transmit and two receive antennas for 40-μsec delayspread, 10-Hz Doppler, and QPSK with differential detection and ½ rateReed-Solomon coding.

A. Other Considerations

1. Battery-Life Awareness

This protocol imposes little additional burden of the battery of themobiles. Although mobiles are asked to broadcast their assigned tones,this is required only of the paged mobiles that have been assigned achannel and are ready to receive/transmit in a super-frame. Thebroadcast occurs only during the assignment portion of that super-frame.

It is important to note that while the present invention has beendescribed in the context of a fully functioning data processing system,those of ordinary skill in the art will appreciate that the processes ofthe present invention are capable of being distributed in the form of acomputer readable medium of instructions and a variety of forms and thatthe present invention applies equally regardless of the particular typeof signal bearing media actually used to carry out the distribution.Examples of computer readable media include recordable-type media suchas floppy disc, a hard disk drive, RAM, and CD-ROM's, as well astransmission-type media, such as digital and analog communicationslinks.

The resulting invention provides an interference-sensing medium accessmethod that meets this condition for asymmetric dynamic channelassignment. The invention can be used with traffic consisting of eithercircuit-switched calls or with packets; the packet length may eitherfixed or variable.

It is to be understood that the above-described embodiments are merelyillustrative of the principles of the invention. Various modificationsand changes may be made thereto by those skilled in the art that willembody the principles of the invention and fall within the spirit andscope thereof.

Appendix I

The interference penalty resulting an interference-sensing schemewhereby only downlink signal measurements are available to determinewhether a channel is used by another base station is estimated byconsidering mobile m, which is covered by a base station E1 in FIG. 12.Potentially m receives interference from vase stations B1, J1, D1, andI1. The interference from each is calculated as follows:

To roughly estimate the penalty on the S/I that would be experienced bythe mobile at location m, we assume that propagation loss coefficient is4. we let R be the cell radius, S the serving signal at location m, andinterf(P) the interference contributed to location m, from source P. Theinterference from each sector and the maximum potential interference IPare calculated as follows:S=α/R ⁴Interf(B 1)=Interf(J 1)=S*R ⁴/(Bm)⁴ =S/16=0.0625*S(Bm)=(Jm)=2*RInterf(D 1)=Interf(I 1)=S*R ⁴/(Dm)⁴ =S/49=0.0625*S(Dm)² R ²+4R ²−4R ²*cos(120°)=(1+4+2)*R ²=7R ²IP=Interf(B 1)+Interf(D 1)*2=0.1658*SIf the target S/I ratio is 10 dB, the total co-channel interference andS/I experienced by mobile m would be:TI=0.1*S+0.1658*S=0.2658*SS/I=S/0.2658*S=3.762=5.75 dBThis represents a 4.25 dB penalty.FIG. 13 illustrates a cloverleaf arrangement for sectorized basestations, whereby abase station is located a vertex of a hexagon(instead of its center, as seen in FIG. 12). A mobile at location mcould potentially receive interference from sectors D1, H1 and I1. Theinterference contributed by each sector and the maximum potentialinterference IP are as follows:Interf(D 1)=Interf(I 1)=S*R ⁴/(Dm)⁴=0.0625*S(Dm)²=25·R ²−9·R ²=16·R ²Interf(H 1)=S*(2R)⁴/(Hm)⁴=0.0256*S(Hm)=5·RIP=(Interf(D 1))*2+Interf(H 1)=0.1506*SIf the design target for the S/I ratio were 10 dB, then the combinedinterference TI and the S/I experienced by a mobile at location m wouldbe:TI=0.1*S+0.1506*S=0.2506*SS/I=S/(0.2506*S)=3.9904=6.01 dBThis represents a 3.99 dB penalty.

1. A method of assigning a downlink channel to a mobile stationregistered with a base station, comprising the steps of: a) turning offpilot tones being transmitted by a plurality of active mobile stationsregistered with the base station, each turned off pilot tonecorresponding to an assigned downlink channel; b) paging the mobilestation with a pending traffic packet from the base station; c)performing interference sensing at the base station to identifyinterference-free downlink channels; d assigning, at the base station, adownlink traffic channel to the mobile station to receive the pendingpackets; e) transmitting the downlink channel assignment from the basestation to the mobile station; and wherein there are a plurality of basestations and steps a)-e) are performed successively for each of theplurality of base stations.
 2. A method of assigning an uplink channelto a mobile station registered with a base station, comprising the stepsof: a) turning off pilot tones being transmitted by the base station,each pilot tones corresponding to an uplink channel assigned to one of aplurality of active mobile stations registered with the base station; b)requesting access from the mobile station for a traffic packet; c)performing interference sensing at the plurality of active mobilestations to identify interference-free uplink channels; d) transmittingfrom each one of the plurality of active mobile stations a list ofuplink channels identified as being acceptably interference-free; e)assigning, at the base station, an uplink channel to the mobile station;and f) transmitting the uplink channel assignment from the base stationto the mobile station.
 3. The method of claim 2, wherein uplink channelsare assigned to a plurality of mobile stations registered with the basestation.
 4. The method of claim 2, wherein there are a plurality of basestations and steps a)-f) are performed successively for each of theplurality of base stations.
 5. A method of assigning an uplink channeland a downlink channel to a mobile station registered with a basestation, comprising the steps of: a) turning off pilot tones beingtransmitted by a plurality of active mobile stations registered with thebase station, each turned off pilot tone corresponding to an assigneddownlink channel; b) turning off pilot tones being transmitted by thebase station, each pilot tones corresponding to an uplink channelassigned to one of the plurality of active mobile stations registeredwith the base station; c) paging the mobile station with a pendingtraffic packet from the base station; d) requesting access from themobile station for a traffic packet; e) performing interference sensingat the base station to identify interference-free downlink channels; f)performing interference sensing at the plurality of active mobilestations to identify interference-free uplink channels; g) transmittingfrom each one of the plurality of active mobile stations a list ofuplink channels identified as being acceptably interference-free; h)assigning, at the base station, a downlink traffic channel to the mobilestation to receive the pending packets; i) assigning, at the basestation, an uplink channel to the mobile station; and j) transmittingthe downlink channel assignment and the uplink channel assignment fromthe base station to the mobile station.
 6. The method of claim 5,wherein downlink channels and uplink channels are assigned to aplurality of mobile stations registered with the base station.
 7. Themethod of claim 5, wherein there are a plurality of base stations andsteps a)-j) are performed successively for each of the plurality of basestations.
 8. A system for assigning an uplink channel to a mobilestation registered with a base station, comprising: means for turningoff pilot tones being transmitted by the base station, each pilot tonescorresponding to an uplink channel assigned to one of a plurality ofactive mobile stations registered with the base station; means forrequesting access from the mobile station for a traffic packet; meansfor performing interference sensing at the plurality of active mobilestations to identify interference-free uplink channels; means fortransmitting from each one of the plurality of active mobile stations alist of uplink channels identified as being acceptablyinterference-free; means for assigning, at the base station, an uplinkchannel to the mobile station; and means for transmitting the uplinkchannel assignment from the base station to the mobile station.
 9. Thesystem of claim 8, wherein downlink channels are assigned to a pluralityof mobile stations registered with the base station.
 10. The system ofclaim 8, wherein there are a plurality of base stations and the systemoperates on each of the plurality of base stations.
 11. A system forassigning an uplink channel and a downlink channel to a mobile stationregistered with a base station, comprising: means for turning off pilottones being transmitted by a plurality of active mobile stationsregistered with the base station, each turned off pilot tonecorresponding to an assigned downlink channel; means for turning offpilot tones being transmitted by the base station, each pilot tonescorresponding to an uplink channel assigned to one of the plurality ofactive mobile stations registered with the base station; means forpaging the mobile station with a pending traffic packet from the basestation; means for requesting access from the mobile station for atraffic packet; means for performing interference sensing at the basestation to identify interference-free downlink channels; means forperforming interference sensing at the plurality of active mobilestations to identify interference-free uplink channels; means fortransmitting from each one of the plurality of active mobile stations alist of uplink channels identified as being acceptablyinterference-free; means for assigning, at the base station, a downlinktraffic channel to the mobile station to receive the pending packets;means for assigning, at the base station, an uplink channel to themobile station; and means for transmitting the downlink channelassignment and the uplink channel assignment from the base station tothe mobile station.
 12. The system of claim 11, wherein downlinkchannels and uplink channels are assigned to a plurality of mobilestations registered with the base station.
 13. The system of claim 11,wherein there are a plurality of base stations and the system operateson each of the plurality of base stations.
 14. A computer programproduct for assigning a downlink channel to a mobile station registeredwith a base station, comprising: a computer readable medium; computerprogram instructions, recorded on the computer readable medium,executable by a processor, for performing the steps of: a) turning offpilot tones being transmitted by a plurality of active mobile stationsregistered with the base station, each turned off pilot tonecorresponding to an assigned downlink channel; b) paging the mobilestation with a pending traffic packet from the base station; c)performing interference sensing at the base station to identifyinterference-free downlink channels; d) assigning, at the base station,a downlink traffic channel to the mobile station to receive the pendingpackets; e) transmitting the downlink channel assignment from the basestation to the mobile station; and wherein there are a plurality of basestations and steps a)-e) are performed successively for each of theplurality of base stations.
 15. A computer program product for assigninga uplink channel to a mobile station registered with a base station,comprising: a computer readable medium; computer program instructions,recorded on the computer readable medium, executable by a processor, forperforming the steps of: a) turning off pilot tones being transmitted bythe base station, each pilot tones corresponding to an uplink channelassigned to one of a plurality of active mobile stations registered withthe base station; b) requesting access from the mobile station for atraffic packet; c) performing interference sensing at the plurality ofactive mobile stations to identify interference-free uplink channels; d)transmitting from each one of the plurality of active mobile stations alist of uplink channels identified as being acceptablyinterference-free; e) assigning, at the base station, an uplink channelto the mobile station; and f) transmitting the uplink channel assignmentfrom the base station to the mobile station.
 16. The computer programproduct of claim 15, wherein downlink channels are assigned to aplurality of mobile stations registered with the base station.
 17. Thecomputer program product of claim 15, wherein there are a plurality ofbase stations and steps a)-f) are performed successively for each of theplurality of base stations.
 18. A computer program product for assigningan uplink channel and a downlink channel to a mobile station registeredwith a base station, comprising: a computer readable medium; computerprogram instructions, recorded on the computer readable medium,executable by a processor, for performing the steps of: a) turning offpilot tones being transmitted by a plurality of active mobile stationsregistered with the base station, each turned off pilot tonecorresponding to an assigned downlink channel; b) turning off pilottones being transmitted by the base station, each pilot tonescorresponding to an uplink channel assigned to one of the plurality ofactive mobile stations registered with the base station; c) paging themobile station with a pending traffic packet from the base station; d)requesting access from the mobile station for a traffic packet; e)performing interference sensing at the base station to identifyinterference-free downlink channels; f) performing interference sensingat the plurality of active mobile stations to identify interference-freeuplink channels; g) transmitting from each one of the plurality ofactive mobile stations a list of uplink channels identified as beingacceptably interference-free; h) assigning, at the base station, adownlink traffic channel to the mobile station to receive the pendingpackets; i) assigning, at the base station, an uplink channel to themobile station; and j) transmitting the downlink channel assignment andthe uplink channel assignment from the base station to the mobilestation.
 19. The computer program product of claim 18, wherein downlinkchannels and uplink channels are assigned to a plurality of mobilestations registered with the base station.
 20. The computer programproduct of claim 18, wherein there are a plurality of base stations andsteps a)-j) are performed successively for each of the plurality of basestations.
 21. A method for medium access control for uncoupled downlinkand uplink channel assignment, comprising: assigning a base station aframe within a periodic super-frame during which it transmits;transmitting narrow-band pilot tones from the base station,corresponding to assigned traffic channels between base station andtheir mobiles; scanning the tones from the base station by mobiles andreporting a list of channels with low interference back to the basestation; said scanning being in a staggered order to avoid contentionfor a same channel between mutually interfering base stations withconcurrent traffic; and selecting at the base station a channel andnotifying the mobile.
 22. A system for medium access control foruncoupled downlink and uplink channel assignment, comprising: a basestation having an assigned frame within a periodic super-frame duringwhich it transmits; said base station transmitting narrow-band pilottones, corresponding to assigned traffic channels between the basestation and mobile stations; a mobile station scanning the tones fromthe base station by mobiles and reporting a list of channels with lowinterference back to the base station; said scanning being in astaggered order to avoid contention for a same channel between mutuallyinterfering base stations with concurrent traffic; and said base stationselecting a channel and notifying the mobile.